Although ozone (O3) occurs naturally in small quantities in the lower troposphere, unhealthy levels of tropospheric O3 are created when high levels of anthropogenic pollutants, such as nitrogen oxides (NOx), and volatile organic compounds (VOCs) are oxidized in the presence of solar radiation. This excess O3 acts as a pollutant and greenhouse gas, contributing to harmful smog that damages human health and ecosystems (Jerrett et al., 2009; Malley et al., 2017; Emberson, 2020) and contributing to higher tropospheric temperatures (Myhre et al., 2013; Stevenson et al., 2013). The relatively short lifetime of O3 in the troposphere (∼3 weeks; Young et al., 2013) means that it is classified as a near-term climate forcer (NTCF), having an important influence on climate over shorter timescales compared to the long-lived greenhouse gases such as CO2. Tropospheric O3 is also an oxidant and a precursor for the hydroxyl (OH) radical (Griffiths et al., 2021). OH (and, by implication, O3) controls the lifetime of methane (Voulgarakis et al., 2013), the second most important anthropogenic greenhouse gas after carbon dioxide (Myhre et al., 2013). Oxidant levels mediate the formation of secondary aerosols such as sulfate and nitrate and play a major role in the aerosol budget and burden, with important consequences for radiative forcing (Shindell et al., 2009; Karset et al., 2018). Understanding how tropospheric O3 changes is important for both future air quality and climate (Turnock et al., 2019).

A multi-model assessment of future changes in tropospheric O3 was made in the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP), using future changes in climate and O3 precursor emissions from the Representative Concentration Pathways (RCPs) (Lamarque et al., 2013). The models participating in ACCMIP projected changes in global annual mean surface O3 concentrations between 2000 and 2030 of ±1.5 ppbv under the different RCPs (Young et al., 2013). More recent single-model estimates by O'Connor et al. (2014) and Kim et al. (2015) predict surface O3 responses across the different RCPs of between -4.0 and +2.0 ppbv by 2050 relative to 2000. The global annual mean tropospheric O3 burden was projected to change by between -18 % and +20 % from 2000 to 2100 under the different RCPs (Cionni et al., 2011; Kawase et al., 2011; O'Connor et al., 2014; Young et al., 2013). Whether tropospheric O3 increases or decreases in future is dependent on the climate mitigation measures and air pollution policies that are implemented. In preparation for the sixth Coupled Model Intercomparison Project (CMIP6), a new set of future pathways was created. Five different socio-economic pathways (SSPs) were developed with centennial trends based on different combinations of social, economic and environmental developments (O'Neill et al., 2014). Different levels of emission mitigation were included within each SSP to meet particular climate and air pollution targets (Rao et al., 2017; Riahi et al., 2017). They incorporate stronger links between socio-economic development patterns and climate change risks than previous assessments and provide better hypothetical scenarios for future projections. The five most widely used scenarios are SSP1-1.9, SSP1-2.6, SSP2-4.5, SSP3-7.0 and SSP5-8.5, where SSP1–SSP5 represent differing socio-economic pathways, and the suffixes 1.9–8.5 indicate the total radiative forcing (W m-2) at the end of the 21st century compared with that before the Industrial Revolution. These pathways provide a good foundation for an assessment of air quality, radiative forcing, ecological environmental effects and human health effects in the future. Many studies have focused on the pessimistic SSP3-7.0 scenario reflecting regional rivalry, and Griffiths et al. (2021) demonstrate that the tropospheric O3 burden increases from 356±31 Tg in the present day to 416±35 Tg in 2100 under this pathway. Liu et al. (2022) show that the tropospheric O3 burden increases by 4 % between 2010 and 2050 under SSP3-7.0. Based on multi-model simulations conducted for the Aerosol and Chemistry Model Intercomparison Project (AerChemMIP), Allen et al. (2020) and Zanis et al. (2022) reveal a global surface O3 decrease in the future under the SSP3-7.0 scenario due to enhanced ozone destruction from higher water vapor abundances under a warmer climate. The sustainability-focused SSP1-1.9 pathway is the scenario that is mostly closely aligned with recent pledges aiming for net zero greenhouse gas emissions, limiting warming to 1.5 ∘C by 2100, but the impacts of this pathway on tropospheric O3 are less well studied and remain unclear.

In east Asia, surface O3 has increased rapidly since 2000 (Lu et al., 2020) and is expected to increase by another ∼10 ppbv by 2050 following the IPCC A1B (Wang et al., 2013), RCP6.0 (Zhu and Liao, 2016) and RCP4.5 (Hong et al., 2019) scenarios. In September 2020, China committed to achieving carbon neutrality by 2060, following the commitments of many developed countries to achieving net-zero emissions by 2050. The effect of these strong mitigation measures on surface O3 has not been explored thoroughly, but the proposed emission pathway to net zero loosely aligns with the SSP1-1.9 pathway. Turnock et al. (2019) showed large reductions of more than 8 ppbv in surface O3 over east Asia by 2050 under this pathway due to large reductions in precursor emissions and CH4. The study also shows that any benefits to surface O3 from reducing local emission sources over east Asia could be offset by intercontinental transport of O3 formed from sources remote to the region and from global CH4 sources. This analysis used an O3 parameterization to rapidly assess changes in O3 and source attribution (Wild et al., 2012; Turnock et al., 2018) which did not account for changes in climate, stratosphere-to-troposphere exchange or chemical regime. Other recent assessments exploring the implications of carbon neutrality in China have suggested that O3 concentrations may decline to 63–94 µgm-3 by 2060 (Shi et al., 2021; Wang and Liao, 2022; Xu et al., 2022). The differences between these results have been attributed to the emission and climate scenarios used. Wang and Liao (2022) also found that the annual mean contribution of southeast Asia to surface MDA8 O3 in China is 3–19 µgm-3, about 2–10 ppbv, and this contribution is reduced in future under the SSP1-1.9 pathway.

While previous studies have quantified possible changes in surface O3 under carbon neutrality, the wider impact on the global tropospheric O3 budget and the changing contributions of different sources remain unclear. In this study, we quantify the changes in surface O3 over east Asia and especially over eastern China, which currently has high anthropogenic emissions, and consider the contribution of different sources based on emissions and climate change under the SSP1-1.9 pathway using the NCAR Community Earth System Model (CESM) with online tagging of O3 and its precursors. We present a self-consistent assessment of the changes in surface O3 associated with changes in emissions and climate, along with the first attribution of these changes. The paper is organized as follows. Section 2 describes the model configurations, experimental settings, O3-tagging method and evaluation datasets. In Sect. 3, O3 and NOx in present-day simulations are evaluated against observations. In Sect. 4, changes in tropospheric O3 under the net-zero scenario are presented. In Sect. 5, the contributions of O3 chemistry and intercontinental transport are discussed under present-day and future conditions. We close with a summary in Sect. 6.

We compared the simulated monthly mean tropospheric column O3 (TCO) with that derived from OMI/MLS for January and July in 2015 (Fig. 2). The model captures the general features of the observed tropospheric column, reproducing the seasonal pattern, with a minimum of 15 DU at 180∘ E in the tropics during January and a maximum of >50 DU in the Northern Hemisphere at mid-latitudes during July. The highest values in the northern mid-latitudes are overestimated in both offline and online simulations, especially during July. In the simulations, TCO was calculated by integrating the O3 from the surface to the tropopause. Some of the differences between the simulated TCO and OMI/MLS may be due to the relatively coarse vertical resolution of the model (26 levels in online simulations and 56 levels in offline simulations). Uncertainty in the satellite dataset (exceeding 5 DU in high latitudes; Ziemke et al., 2011) might also contribute to these differences. The accuracy of the emission inventory also affects the simulation results, especially at the surface. The global (60∘ S–60∘ N) annual mean tropospheric O3 columns from the offline and online simulations are 29.0 and 32.3 DU, respectively, which match those from OMI/MLS (31.7 DU) and the ACCMIP models' mean values (30.8 DU; Young et al., 2013) well. The online simulated tropospheric ozone column global annual average is the highest due to the coarser vertical resolution in the online simulation (Lamarque et al., 2012).

As shown in Fig. 3, surface O3 shows substantial seasonal variations with low concentrations in winter and high concentrations in summer. The spatial distributions of simulated surface O3 concentrations match the observations well. The online simulated surface O3 (ppbv) is overestimated by 9.2 ppbv on average in winter, especially in Mongolia, north and middle of China, Korea, and Japan, while the offline simulation is much closer to the observation with a bias of 3.6 ppbv. The coarser resolution of the online model is likely a reason for its higher bias. The comparison of simulated surface O3 (ppbv) with EANET observations shows that the simulations reproduce the seasonal variations at these 12 sites (Fig. S2 in the Supplement). In general, the performance of these simulations is very similar to those from other chemical model studies (Li et al., 2019; Young et al., 2018).

An overview of the global model diagnostics for the simulation experiments is given in Table 4. The tropospheric O3 burden and budget terms for present-day conditions in this study match previous results well. Under net zero, the chemical production decreases from 5038 to 3392 Tg(O3) yr-1, and the chemical loss decreases from 4641 to 3311 Tg(O3) yr-1. The net chemical tendency of tropospheric O3 (NetChem in Table 4) drops substantially, decreasing from the current 397 to 81 Tg(O3) yr-1 due to the large reduction in O3 precursor emissions (Table 2). This results in an increase in the lifetime of tropospheric O3 from 20 to 22 d. The residual term, which principally reflects net transport from the stratosphere, increases from the current 595 to 626 Tg(O3) yr-1. The global tropospheric O3 burden decreases by about 20 %, from 316 to 247 Tg, bringing it close to the mean burden of 239±22 Tg estimated for the pre-industrial period (Young et al., 2013; Griffiths et al., 2021). The burden of O3 of stratospheric origin in the troposphere (O3S) increases from 69 to 77 Tg. This increased stratospheric contribution may be due to the enhancement of stratospheric circulation and increased stratosphere–troposphere exchange caused by climate change (Sudo et al., 2003; Lu et al., 2019) and has been seen in previous studies (e.g., Zanis et al., 2022). In addition, the longer chemical lifetime allows stratospheric O3 to persist for longer in the troposphere, enhancing the stratospheric contribution. Compared with pre-industrial conditions (Griffiths et al., 2021; Table 4), the net chemical production rate is slower, while the stratospheric contribution is higher. This may indicate that anthropogenic influence is somewhat weaker than that in the pre-industrial. Compared with other SSP scenarios, particularly the much-studied SSP3-7.0 pathway (Liu et al., 2022; Griffiths et al., 2021), SSP1-1.9 provides a more positive perspective on the opportunities for controlling future tropospheric O3 and the benefits for air quality.

Over east Asia, the net photochemical production of tropospheric O3 also decreases significantly, from the current 227 to 137 Tg(O3) yr-1 under net zero, but the reduction is less than the global average, and this is attributed to the higher emissions and smaller reductions in precursors over east Asia. The negative residual budget term for east Asia indicates that the production is larger than the sink, and the total contribution of vertical and horizontal transport from outside of east Asia is negative. This indicates that there is net outflow from east Asia with transport of tropospheric O3 to other regions, and this outflow is weakened in the future, from 89 Tg(O3) yr-1 under present-day conditions to 38 Tg(O3) yr-1 under net zero. The tropospheric O3 burden in east Asia decreases from 25 to 19 Tg, while the burden of O3 from the stratosphere increases slightly from 5 to 6 Tg. The tropospheric O3 lifetime in east Asia is 15 d, slightly lower than the global average due to the faster photochemical processing under relatively high anthropogenic emissions. But the increase of ∼2 d matches that of the global average.

The changes in surface O3 over east Asia between 2015 and 2060 in winter and summer are shown in Fig. 4. The left panels show the changes in surface O3 under net zero (online-NZ minus online-PD), which include the effects of climate change and emissions changes. The climate change in this study includes changes in atmospheric parameters (air temperature, relative humidity, atmospheric circulation, etc.) from the free run of atmospheric simulation experiments (online). The climate change under SSP1-1.9 is much weaker than under other pathways, and the change in global surface air temperature in this study is not significant (Fig. S3 in Supplement). Over east Asia, the surface air temperature is increased by an annual average of 0.2 ∘C. The middle panels show the changes in surface O3 under the effect of emission changes only (offline-NZ minus offline-PD). The right panels show the residual changes in surface O3 which reflect the effect of climate change but are also influenced by differences in the model setup between the online and offline simulations (left panels minus middle panels). Surface O3 decreases in east Asia under net zero, with a mean reduction of 7.7 ppbv in winter and a greater reduction of 16.2 ppbv in summer. Turnock et al. (2019) estimated an annual mean reduction of 8 ppbv in 2050 under the SSP1-1.9 pathway, slightly less than we find here. However, we have used the more stringent DPEC Ambitious-pollution-Neutral-goals emission scenario for China rather than the standard SSP1-1.9 pathway, and we note that anthropogenic NO emissions in China are 1.5 Tg (NO) yr-1 lower in this scenario than those in SSP1-1.9. Surface O3 over eastern China and South Korea increases in winter in these scenarios, driven by the reduction in emissions (left and middle panels). This increase in surface O3 is caused by a weakening of titration under lower regional NO emissions in the future. The influence of climate change on surface O3 is relatively weak and leads to an increase in surface O3 in most parts of east Asia (right panels). This is partly due to enhanced vertical circulation leading to an increased contribution from stratospheric O3 (Akritidis et al., 2019; shown in Fig. S4) and the photochemical change under warmer climate (Zanis et al., 2022). Xu et al. (2022) also showed that emission reduction is far more effective than climate change in improving air quality (PM2.5 and O3) over east Asia under a carbon-neutral reduction pathway. Here, we will use tagging simulations to quantify the contributions of different sources to surface O3 changes over east Asia, especially over eastern China where surface O3 increases in winter and decreases in summer.

It can be seen from the vertical distribution of O3 and circulation (shown in the first panels of Fig. 5) that the O3 concentration increases with altitude under present-day conditions. At the same altitude, the O3 concentration is higher in middle and high latitudes than in low latitudes. In winter, there is a strong net descent of air over eastern China (30–40∘ N), which weakens in spring and turns to updraft in summer. These may be due to the weakened Brewer–Dobson circulation and strengthened convection (Butchart, 2014; Wild and Akimoto, 2001). As shown in the second panels of Fig. 5, there is a net decrease in tropospheric O3 in future, with an increase only seen near 30∘ N very close to the surface. In summer, the reduction in tropospheric O3 is greatest, especially near the tropopause where it exceeds 30 ppbv. In addition, due to the temperature increase and circulation enhancement in the future, the tropopause height increases, especially in the mid-latitude region in winter, where the increase is about 7 hPa. As seen from the third panels of Fig. 5, the reduction in emissions from aircraft (NO emissions in Fig. S1) leads to a reduction in O3 production, and the O3 concentration near the tropopause decreases substantially in the future. However, other factors such as climate change (the fourth panel in Fig. 5) lead to increases in tropospheric O3 by 2060.

Surface O3 shows substantial seasonal variations over east Asia, with a peak in spring, as shown in Fig. 6a. It reaches a maximum (56 ppbv) in March and is lowest (41 ppbv) in August under present-day conditions. Under net zero, the concentration of surface O3 is lower throughout the year, and while the peak is still in March, the mixing ratio drops to 43 ppbv. The decrease is greatest in July at 16 ppbv, which reflects weaker chemical production in summertime under lower future emissions (Fig. 6e). In contrast, surface O3 over eastern China is highest (71 ppbv) in July and lowest (21 ppbv) in December under present-day conditions (Fig. 6b). Under net zero, surface O3 increases in winter and decreases in summer, and the peak shifts from July to May due to the changes in O3 precursor emissions (Bowman et al., 2022). This shifts the seasonal peak from summer towards spring, when it is more greatly influenced by stratosphere–troposphere exchange. The decrease is highest in July, as seen over the wider east Asian region, but it is twice as large at 34 ppbv, reflecting the stronger present-day emissions over eastern China. There is a substantial increase in O3 in January of 12 ppbv, reflecting reduced titration by NO. The concentration of surface NOx decreases by more than 60 % and by an even larger factor in winter (∼90 %, 14 ppbv), and its seasonal variation is reduced, which accounts for the reduction in anthropogenic emissions (Fig. 6d). In terms of the O3 chemical budget, local chemical production and destruction are both reduced in the future. The peak in net O3 chemical production still occurs in summer, which highlights that photochemical processes continue to dominate the seasonal variation of surface O3 in eastern China in future (Fig. 6f). However, the net chemical destruction that currently occurs in winter is replaced with a small net O3 production, reflecting the reduced titration of O3 by NO under future emissions, which are very greatly reduced under net zero (Liu et al., 2022, 2023).

We quantify the contributions of regional transport and stratospheric input to surface O3 on a monthly basis in Fig. 7. In the present day (Fig. 7a), we find that the contribution of anthropogenic NO emissions from east Asia (EAS) is highest, especially in summer, when it reaches 30 % (12 ppbv in Fig. 8). The total contributions from anthropogenic NO emissions outside east Asia (EAS_out, without ocean) are 33 % (16 ppbv) on average, with little seasonal variation, and these are highest over south Asia (SAS), accounting for 12 % (6 ppbv). The contribution from biogenic NO emissions from soils (BIO) is also important, exceeding 10 % (5 ppbv in Fig. 8) in summer. The contributions from the ocean (OCN) show little seasonal variation, contributing 15 % (6 ppbv). Under net zero (Fig. 7c), the anthropogenic contribution from east Asia drops dramatically to 14 % (4 ppbv in Fig. 8) in summer due to the reduced emissions of O3 precursors. The total contributions from anthropogenic NO emissions outside east Asia decrease to 28 % (10 ppbv) on average and to 7 ppbv in summer (in Fig. 8). The contribution of biogenic sources is enhanced and forms the dominant contributor to surface O3 under net zero, especially in summer at ∼40 % (9 ppbv in Fig. 8). The emissions from biogenic sources do not vary from year to year in this study. The enhanced contribution of biogenic sources is mainly due to the increased O3 production efficiency, which is a consequence of lower O3 precursor concentrations (Kleinman et al., 2002; Zaveri et al., 2003). The contribution of oceanic sources decreases to 4 % (1 ppbv) due to reduced emissions from shipping. The contribution from stratospheric O3 (STR) is highest in March (26 %, 14 ppbv) and lowest in August (7 %, 3 ppbv) under present-day conditions. Under net zero, the highest contribution is increased to 39 % (17 ppbv), and the lowest contribution is also increased to 12 % (3 ppbv). This may be due to enhanced stratospheric circulation, slower photochemical loss and a longer lifetime of O3 in the troposphere, allowing greater transport of stratospheric O3 to the ground.

Over eastern China (Fig. 7b), the contribution from east Asian anthropogenic sources is highest, especially in summer, when it exceeds 70 % (43 ppbv, shown in Fig. 8). The total contributions from anthropogenic NO emissions out of east Asia are 16 % (6 ppbv) on average and 4 ppbv in summer (in Fig. 8). Biogenic and oceanic sources make a smaller contribution over this region, only 6 % (3 ppbv) and 10 % (5 ppbv) on average, respectively. Under net zero (Fig. 7d), the contribution of east Asian anthropogenic sources drops to 42 % (16 ppbv) in summer but remains the dominant source of surface O3 in eastern China. The total contributions from anthropogenic NO emissions outside east Asia show little change. The contribution of biogenic sources is enhanced, especially in summer, reaching 40 % (14 ppbv in Fig. 8), close to the contribution from east Asian sources. The stratospheric contribution is highest in early spring (25 %, 11 ppbv) and lowest in summer (2 %, 1 ppbv). Under net zero, the stratospheric contribution is enhanced to 40 % (17 ppbv) in March and 3 % (1 ppbv) in summer, similarly to the seasonal contributions over east Asia. In addition, the high NO concentration in heavily urbanized eastern China has a titration effect on O3, but the strong future decreases in NO weaken this effect, reducing the loss of stratospheric O3 and of O3 from local sources. Overall, surface O3 shows substantial decreases through much of the year, and the local contribution is reduced, which highlights the beneficial role that net-zero policies may have for controlling surface O3 pollution in China.

We quantify tropospheric O3 budgets, spatiotemporal distributions of future surface O3 in east Asia and regional O3 source contributions for 2060 under a net-zero scenario using the NCAR Community Earth System Model (CESM) and online O3-tagging methods. The simulated monthly mean global tropospheric column O3 and surface O3 mixing ratios over east Asia capture the general features in observations well under present-day conditions. The offline simulations perform better than online simulations as the nudging provides a closer match to observed meteorological conditions. The tropospheric O3 burden and budget terms under present-day conditions in this study also match those of previous model studies well.

The simulated tropospheric O3 burden is likely to decrease from 316 Tg under present-day conditions to 247 Tg by 2060 under the net-zero scenario. This brings it close to that found in previous studies under preindustrial conditions of 239±22 Tg (Young et al., 2013). Future tropospheric O3 chemical production and loss are both reduced, and the net chemical tendency decreases from 397 to 81 Tg(O3) yr-1. The contribution of stratospheric O3 increases from 69 to 77 Tg due to the enhancement of atmospheric circulation and the increased stratosphere–troposphere exchange that is caused by climate change and the longer chemical lifetime of stratospheric O3 in the troposphere under decreased anthropogenic emissions of pollutants. The mean tropospheric lifetime of O3 is increased by 2 d (∼10 %). Over east Asia, one of the highest anthropogenic emission regions, the O3 burden decreases from 25 to 19 Tg, and the net chemical tendency drops from 227 to 137 Tg(O3) yr-1. East Asia is a region of net O3 production, and the outflow is expected to decrease from 89 to 38 Tg(O3) yr-1. The burden of O3 from the stratosphere increases from 5 to 6 Tg. The lifetime of tropospheric O3 over east Asia is shorter than the global average at ∼15 d due to the high anthropogenic emissions, but it increases by 2 d, similarly to the global mean. Compared with other SSP scenarios, particularly the much-studied SSP3-7.0 pathway, SSP1-1.9 provides a more positive perspective on the opportunities for controlling future tropospheric O3 and the benefits for the improvement of air quality.

Regional average surface O3 decreases throughout the year over east Asia, with the highest decreases in summer (16 ppbv) in the future under the net-zero scenario. Over eastern China, the peak in surface O3 shifts from July to May. Surface O3 decreases strongly in July (34 ppbv) and increases in winter, especially in January at 12 ppbv. The increased O3 in winter is caused by reduced titration of O3 by NO associated with lower anthropogenic NO emissions and by enhanced stratospheric input. The tropospheric O3 over most regions decreases due to the large decrease in O3 precursor emissions. Climate change leads to only a small increase in tropospheric O3 under this scenario. Local anthropogenic emissions play a dominant role in controlling O3 changes over east Asia in summer, but this will drop substantially from 30 % in the present day to 14 % under net zero. The contribution of biogenic sources is enhanced and forms the dominant contributor to future surface O3, especially in summer at ∼40 %. Here, this enhanced contribution of biogenic sources is due to increased O3 production efficiency associated with reduced O3 precursor concentrations, but this may be underestimated if biogenic emissions also increase in future as expected. The lower extent of climate change under SSP1-1.9 leads to relatively little impact on tropospheric O3 under net zero, while the emission reductions associated with net-zero policies are sufficient to mitigate surface O3 pollution over east Asia, especially in summer.

The combined emissions and O3-tagging method used here provide a reliable way to quantify the changes in tropospheric O3 and its sources in future under a net-zero scenario. The results of this study clarify the separate impacts of climate change and emissions on tropospheric O3 changes over east Asia and highlight the significance of controlling O3 precursor emissions along the net-zero scenario, especially anthropogenic emissions. The reduction in anthropogenic O3 precursor emissions should be the most effective way to control the increase in tropospheric O3, and this requires joint efforts on a global scale.